A dielectric waveguide is a physical structure configured to confine electromagnetic radiation transversely so as to guide the radiation in a preferred direction. As shown in FIG. 1C, this preferred direction is referred to as the optical axis 114.
A particular class of such waveguides is characterized as planar waveguide amplifiers. Such planar waveguide amplifiers include at least three stacked layers of materials (e.g., cladding-core-cladding), with at least one layer (e.g., core) having a different refractive index than other adjacent layers. The cladding layers may or may not be equal in size or material with the only requirement that their respective refractive indexes are smaller than the core index. One or more of the layers could for example be air. In operation, electromagnetic radiation is confined substantially within a core region of such a planar waveguide by total internal reflection (TIR), as can be realized by providing a middle layer having a refractive index that is larger than the refractive indexes of the surrounding layers or regions. The surrounding regions of such structures can be referred to as a cladding with respect to the core.
An example of a planar waveguide 100 is illustrated in FIG. 1A, having a rectangular planar core layer 102 sandwiched between upper and lower rectangular planar cladding layers 104a, 104b. Propagating waveguide modes in such structures 100 can be excited by injecting electromagnetic radiation into the core layer 102 from an edge of the device. In rectangular configurations, the core 102 represents a rectangular guiding plane bounded by two opposing end facets 106a, 106b (generally 106) spaced apart along a longitudinal axis (e.g., an optical axis 114) and two opposing side facets 108a, 108b (generally 108) spaced apart along a transverse axis, such that the side facets 108 are parallel to the longitudinal axis. Electromagnetic energy (e.g., a signal) directed along the longitudinal axis, generally enters and exits such a rectangular planar waveguide slab through the end facets 106.
Ideally, only signal beams would propagate within the core layer 102 and those beams would propagate along a trajectory parallel to the optical axis 114. However, both signal beams and fluorescence beams typically propagate within the core layer 102 and there are typically many beams of each type which do not propagate parallel to the optical axis 114. The angles of various signal and/or fluorescence beams relative to the optical axis 114 and/or planar core layer are referred to as beam angles (α as shown in FIG. 1C). In-plane beam angles (i.e., parallel to the planar core layer 102 but angled relative to the optical axis are generally expressed as α. Out-of-plane beam angles (i.e., parallel to the optical axis 114 but angled relative to the planar core layer 102) are generally expressed as γ. The angles of various signal and/or fluorescence beams relative to the side and/or cladding layers are referred to as angles of incidence ( α as shown in FIG. 1C). In-plane and out of plane angles of incidence are generally expressed as α and γ respectively.
Advantageously, such dielectric waveguide devices can be configured as amplifiers, exhibiting high-gain, high-power and high-energy capabilities. For example, planar waveguide amplifiers include a so called gain medium in the guiding layer. The gain medium is configured to provide amplification to injected electromagnetic radiation, for example, through a process generally known as stimulated emission. A pump source provides energy to the gain medium, urging electrons or molecules of the gain medium into an excited state. Interaction of injected electromagnetic radiation (i.e., signal photons) with the excited (i.e., pumped) gain medium results in the generation of photons through transitions of electrons or molecules from the excited energy state to a lower energy state. Thus, electromagnetic energy injected into one end of such a rectangular planar waveguide amplifier will travel along the core region, being amplified along the way by the generation of additional photons through stimulated emission.
The signal gain achievable in such devices depends at least in part on the available population of excited electrons or molecules. Unfortunately, in a pumped gain medium some photons are emitted spontaneously (referred to as fluorescence), independent of the signal field. A schematic example of a guided fluorescence photon 112 is shown in FIG. 1B. Spontaneously emitted fluorescence photons can occur at random locations within the gain medium, having random directions and random polarizations. Such fluorescence photons traveling within a pumped gain medium can give rise to further unwanted photons through stimulated emission (e.g., amplified spontaneous emission, or ASE) and are generally referred to as “parasitic” or “parasitic modes.” It should be appreciated that stimulated photons take on the same phase, frequency, polarization and direction characteristics as the stimulating photons, regardless of whether the stimulating photons are signal photons or fluorescent photons. Accordingly, such parasitic modes (e.g., ASE) reduce the number of excited electrons or molecules available for amplifying the signal, thereby reducing the achievable gain for any given pumping power.
There are two classes of fluorescence photons, (1) photons which are emitted under shallow angles relative to the waveguide plane such that they remain confined/guided by the waveguide, and (2) photons which are emitted under large enough angles relative to the waveguide plane, such that they can reach the cladding directly. Guided fluorescence photons are particularly damaging. In their worst form, they can form in-plane TIR (TIR=Total internal Reflection) loops (e.g., the loop contains reflections from the two side facets 108 and two end facets 106). Less severe, but still damaging to the performance of a planar waveguide amplifier are guided fluorescence photons which travel from end-facet to end-facet of the waveguide and possibly reflecting off the long sides while doing so.
Typically, gain in a pumped gain medium is limited at low pump power by the fluorescence lifetime of the excited state and, at higher powers, by parasitic modes. Among these parasitic modes are modes which reflect off the sides of the amplifier structure (reflective modes) and those where the light propagates directly from end-facet to end-facet (ballistic modes). Of the reflective modes, it is the modes where all reflections occur under total internal reflection (TIR) conditions which are particularly damaging to amplifier performance.
It is possible for amplified spontaneous emission originating near an end facet and travelling through the full length of the amplifier to ultimately limit the gain achievable at a given pump power. As shown in FIG. 2, a spontaneously emitted photon 212 in a waveguide amplifier 200 can give rise to parasitic modes in the form of total internal reflection (TIR) loops. In the illustrative example, a photon 212 is generated within a core region 202 of a planar rectangular waveguide amplifier 200. The photon 212 is directed generally along a ray 213, which in the illustrative example is guided through the core 202. Such guided photons 212 propagate through the core 202 until they encounter one of the side facets 208a, 208b or end facets 206a, 206b. In the illustrative example, the photon 212 encounters the right side facet 208b at an in-plane (i.e., within the plane of the core 202) angle of incidence α. In at least some instances (e.g., if α is larger than a threshold angle of incidence for a core-air interface), the photon 212 is fully reflected back into the core 202. The threshold angle of incidence for the core-air interface can be determined as
            α      _        =          asin      ⁡              (                              n            air                                n            core                          )              ,wherein ncore is the refractive index of c is the planar core layer 202 and nair is the refractive index of air.
In the illustrative example, the photon 212 traverses a width of the core 202 and encounters the left side facet 208a at substantially the same in-plane angle of incidence α, because both side facets are parallel and aligned with a longitudinal axis. The process may repeat until the photon ultimately encounters one of the end facets 206a, 206b at an in-plane complementary angle of incidence α=90− α, resulting from the rectangular geometry of the waveguide amplifier 200. In at least some instances, the photon 212 is once again, reflected back into the core 202 at the end facet-air interface, as shown. In such circumstances, the photon 212 is effectively trapped within the rectangular waveguide amplifier 200 in a TIR loop. TIR loops can be supported when α, exceeds the critical angle for TIR. For example for rectangular YAG in air, this occurs for 33.3 deg< α<56.7 deg, or in general φ1≦ α≦φ2. For rectangular waveguides made from some material or combination of materials, TIR loops exist when the threshold angle φ1 for the material interface is less than 45°. If TIR loops are allowed by the prevailing materials, and since the direction and orientation of spontaneously emitted photons 212 occur randomly, it is unavoidable that some fluorescence photons 212 will be trapped in TIR loops.
Thus, photons 212 of a TIR loop are effectively trapped within the gain medium of the core 202, making endless passes through the core 202. When the core 202 is configured to provide gain, each trip through the core 202 stimulates additional photons through the process of stimulated emission as described above. In such scenarios, the stimulated photons have the same direction and thus, further contribute to further growth of the power in the TIR loop, thereby continuously reducing the available signal amplification power.
One approach for reducing non-guided ASE in waveguide amplifiers is to provide a large cladding volume relative to the core 202. The cladding can be used to draw fluorescence photons out of the gain region, thereby reducing amplified spontaneous emission.
An example of one such device, which may be referred to as a five-part device, is shown in FIG. 1A and FIG. 1B. In addition to the upper and lower claddings 104a, 104b described above, the planar waveguide amplifier 100 includes left and right side claddings 110a, 110b (generally 110). Each side cladding 110 can be adjoined to a respective one of the side facets 108. As shown in the cross section of FIG. 1B, a spontaneously emitted photon 112 occurring within a central region of the core 102, can be guided through the core 102 until it encounters one of the side facets 108, upon which it may exit the core 102 and enter the side cladding 110. Thus, such guided fluorescence can be effectively “drained away” from the planar waveguide core through the side claddings 110. Unfortunately, the addition of such side claddings 110 substantially adds to complexity and associated fabrication costs because side claddings must generally be added by additional process steps, after the planar waveguide itself has been fabricated.
Other attempts at reducing ASE without requiring side claddings 110 involve providing a diffuse finish along the side facets 108. Such a diffuse finish allows at least a fraction of ASE photons impingent upon the side facet 108 to exit the core 102. Unfortunately, the diffuse finish does not discriminate between ASE photons and signal or pump photons. Thus, using diffuse finishes is a less desirable option because it reduces the amplifier efficiency that would otherwise be achievable with polished facets.